Calculation of Parameters for the Astatic Magnetometer

Abstract

The sensitivity and the signal-to-noise ratio of an astatic magnetometer are proportional to the intensity of magnetization of each magnet in the suspended system. Hitherto the intensity of magnetization has been calculated using values of demagnetization factors for ellipsoids and have been applied to astatic design incorporating rectangular or cylindrical magnet forms.In the present paper, the magnetizations of both rectangular and cylindrical magnets have been calculated again using more exact values of the demagnetization factors for rectangular and cylindrical shapes, respectively. Then the calculated parameters for the astatic magnetometer in this study are compared with the previous calculations to show the effect of the demagnetization factors for ellipsoids on the theoretical calculations in the astatic design incorporating rectangular or cylindrical magnets. It is shown that the theoretical values of both sensitivity and signal-to-noise ratio of an astatic magnetometer are sensitivity (ellipsoid approx.)/sensitivity (exact shape) = 1.5; and signal-to-noise (ellipsoid approx.)/signal-to-noise (exact shape) = 2.0. These are lower than the values in previous calculation in the case of longitudinal magnets. But this previous assumption is not significant for transverse magnets, say FEROBA 111.

Instruments and techniques in paleomagnetism and rock magnetism

“…However, in practice a useful limit is set by the increase of a in small systems. Whether this is taken into account [Roy, 1963;Deutsch et al, 1967] or neglected (Collinson, MP 47) the basic requirement is a magnet which combines maximum magnetic moment with minimum moment of inertia [see also Ispir, 1971]. During the 1960's, several new magnetic materials were produced which enabled the ratio P/1o to be improved; among these were ceramic barium ferrites (Magnadur II and III, Indox V), Platniax II (platinum-cobalt), and Columax, an improved form of Alcomax III (basically an iron-alumnium-cobalt-nickel alloy).…”

“…This problem is analogous to that of producing uniform fields, and Thellier's systems include four equal coils separated by the same distance from each other and a 'double' Helmholtz set with one pair twice the radius of the other but with the same area turns. This and other systems for producing uniform fields have been analyzed by Ispir [1969].…”